Re: [avr-gcc-list] Optimization around mul and restoring r1

[Georg-Johann Lay]

David Brown schrieb:
> On 01/10/13 09:41, Simon Kirby wrote:
> > Hello!
> >
> > How difficult would it be to convince avr-gcc to not re-zero r1 instantly
> > after mul, but delay it until a zero is actually needed? For example,
> > see this actual avr-gcc output:
> >
> >         Vneutral = (uint16_t)Vbus * t >> 8;
> > ->
> >         lds     r25, 0x20FD
> >         mul     r24, r25
> >         movw    r24, r0
> >         eor     r1, r1
> >         sts     0x205C, r25
> >
> > Could be just:
> >
> >         lds     r25, 0x20FD
> >         mul     r24, r25
> >         sts     0x205C, r1
> >         eor     r1, r1
> >
> > Saving one whole cycle! ;)
> In general, it would be a good thing to delay the zeroing of r1, and it
> is worth searching the issue database for previous issues and filing a
> new "missed optimisation" issue if it is not there already:

IMHO everyone working on the avr back-end is aware of this problem.

Johann